Wavelength division multiplexed, WDM, optical communication systems utilising coherent detection are attractive for their capability to recover linear impairments like chromatic dispersion and polarization mode dispersion that can be effectively mitigated by coherent detection and subsequent digital signal processing. A WDM optical signal is degraded by optical noise accumulation and impairments during propagation. Amplified spontaneous emission, ASE, noise accumulation is unavoidably related to the optical amplification, performed via erbium-doped fibre amplifiers, EDFA, or Raman amplifiers, which the WDM optical signal undergoes.
In existing systems, to optimise optical signal transmission quality the optical signal launch power is usually set to maximize the optical signal-to-noise ratio, OSNR, whilst keeping nonlinearities under a preselected tolerable threshold. G. Bosco et al, “Performance prediction for WDM PM-QPSK transmission over uncompensated links”, in Proc. OFC 2011, paper OTh07 (2011) report that the variance of the nonlinear noise for a given transmission link is well approximated as AP3 where P is the channel power and A is a constant which depends on system parameters and can be obtained numerically or analytically. The performance of the system can therefore be characterized by a “total” signal to noise ratio, which can be written as1/SNR=1/SNRlin+1/SNRNL  Equation 1Where 1/SNRlin is the inverse of the linear noise limited by ASE and implementation penalty. It can be further modelled as:1/SNRlin=1/SNRASE+KTRX  Equation 2as reported by Vacondio et al, “On nonlinear distortions of highly dispersive optical coherent systems”, Optics Express, January 2012, vol. 20, no. 27. The first term can be obtained from known span losses, launch powers and amplifier noise figures, and KTRX is a parameter that models the practical implementation of the transmitter, the receiver and the filter chain of the link. KTRX is known to the equipment manufacturer and the lower its value the better (it is 0 for an interface matching the ASE-limited performance). The second term consists of the nonlinear noise variance divided by the channel power, and has a slope of about −2 dB/dB with respect to the channel power.
A common approach to maximising optical signal transmission quality in coherent optical transmission systems is to simulate the effective Q factor (the bit error rate, BER, expressed through the inverse of the erfc( ) function) as a function of the launch channel power, and find the launch channel power for which the maximum Q is achieved for the system. An advantage of this solution is that is can be performed before the real system is deployed. A disadvantage is the large computational effort that is required to run the simulation of the propagation and detection of the optical signal.